are several other ranges, for example, the Park Range, the Sangre de Cristo, the Sawatch, and the San Juan ranges (Figure 1). Main east-west highways crossing the Colorado Rockies, shown by heavy black lines on the map, lead through numerous passes over which millions of people travel every year (Figures 1 and 2). Among the best known are Fall River (9) and Milner (8) on U. S. Highway No. 34; Berthoud (15) and Rabbit Ears (5) on U. S. No. 40; Love-